The aim of this project to study the genetic mechanisms involved in the evolution of sex chromosomes. The results will have general importance for the understanding of sex chromosomes. Many human genetic disorders are caused by sex chromosome abnormalities, and a better understanding of the factors involved in the organization of sex chromosomes will help in their elucidation. The genetics of the sex chromosomes of a flowering plant, Silene latifolia, will be studied, using natural molecular variation as a source of markers. Once variable loci have been found, crosses will be set up to study their linkage relations, and to identify the sex-linked loci among them. Given such loci, genetic studies of a plant sex chromosome will be possible. We will determine which loci have active copies on both the X and Y chromosomes, and whether they are present in the pairing or differential segments of the sex chromosomes. We will also examine mutant sex chromosomes to determine in what ways the genetic map is altered when different mutants sex phenotypes arise. We shall also study the levels of expression of any loci which are hemizygous in males, to determine whether dosage compensation happens in a plant species. Families of transposable elements from Drosophila americana will be cloned and their distribution in the genome characterized by in situ hybridization of element probes to polytene chromosomes. The aim is to determine whether or not transposable elements are more abundant on the neo-Y chromosome of this species than in the euchromatin of the other chromosome arms, and on the homologous arm of its relatives, which is autosomal. Such an accumulation is expected both on theoretical grounds, and from empirical evidence that elements tend to acuminate where recombination is restricted.